Organic matter cycling in hypoxic environments: the role of oxygen availability and benthic faunal communities

Continental margins receive significant amounts of organic matter (OM) from terrestrial and marine sources, and it is estimated that over 80 % of all organic carbon (OC) preservation in marine sediments takes place in these areas. The least well understood aspect of OM cycling and burial in marine sediments is the role of benthic fauna living on and in the sediments. Seafloor communities influence marine sedimentary OM cycling and burial via a number of activities including digestion, bioturbation or burrowing, respiration, irrigation and ventilation, and through microbial stimulation. In turn, several factors are known to influence benthic biological processing of OM: oxygen, OM quality and quantity, temperature, and faunal size and abundance. Thus, dynamic relationships exist between benthic faunal communities, sediment geochemistry and oxygen availability, yet these remain poorly understood or quantified. Results from previous studies indicate that benthic communities usually intercept and rapidly ingest most of the OM flux delivered to the seafloor but that the response varies between faunal groups such as bacteria, foraminifera and megafauna. In this study, whole-community experiments were conducted at sites with a natural range of biogeochemical characteristics, across the Indian Margin oxygen minimum zone (Arabian Sea) and the Gotland Basin (Baltic Sea). In order to assess how short-term faunal OM processing varies with oxygen availability, shipboard incubation experiments were conducted under both ambient and manipulated oxygen concentrations. 13C and 15N-labelled phytodetritus (chlorella) was added to incubations to mimic seasonal OM fluxes. The added isotopically-labelled OM was traced into different pools including fractions of the faunal community, sediments, pore waters and overlying waters. Faunal uptake of added OM was determined by isotopic enrichment of organic carbon (δ13OC) and nitrogen (δ15Norg) of faunal body tissue, bacterial assimilation was assessed through 13C analysis of specific bacterial polar lipid fatty acids (PLFAs), and respiration was measured from 13C enrichment of dissolved inorganic carbon (DIC, ΣCO2). Results suggest that respiration is the dominant fate of OM at the seafloor in marine sediments. Organic matter processing is dominated by bacteria or foraminifera where oxygen is depleted and organic matter quality is low. Biological sedimentary organic matter processing is evenly shared between macrofauna, foraminifera and bacteria where oxygen is not limited and organic matter quality is high. This study is one of the first to conduct such isotope tracing experiments in conjunction with oxygen manipulation. Oxygen availability was found to be the over-riding control on organic matter preservation and short term cycling at the seafloor in both regions